1. Field of the Invention
The present invention relates generally to an ink jet printing apparatus and an ink jet printing method using the ink jet printing apparatus and, more particularly, to an ink jet printing apparatus, an ink jet printing method and a printing system employing a different printing method according to the number of colors of ink to be used for printing.
2. Description of the Related Art
With the recent development and spread of digital cameras, there is a need for an image quality comparable to that of silver-salt photos even with an ink jet printing apparatus capable of producing an output of a shot image onto a print medium, such as paper or the like, easily in home-use environment. Conventionally, image quality in print results of color photo tone images is therefore enhanced by carrying out printing using six colors of ink of low-concentration ink of a light cyan and a light magenta added to the ink of four colors of cyan, magenta, yellow, and black.
There is also a trend, in which digital cameras of a single reflex type are marketed at relatively low prices. Ink jet printing apparatuses are therefore used for printing monochrome photo tone images as well as color photo tone images Generally speaking, black ink is mainly used in printing of the monochrome photo tone image. A monochrome image using black ink only is, however, recognized as being slightly tinted with color. For this reason, cyan (or magenta) and yellow are used for correcting color tone, in addition to the black ink that serves as a basic tone of the monochrome image. Further, to lessen a granular impression in low and middle gradations, it is practice to create gray using light cyan and yellow inks. That is, a multi-color printing is also performed by using a plurality of chromatic colors in addition to black as an achromatic color in order to enhance image quality of the monochrome photo tone image. A dot formed by ejected ink may land on a position deviated from an intended position. If this happens, an intended color is not formed. If a color other than an intended achromatic color serving as the basic tone in printing of the monochrome image is evident in a printed image, that particular point appears inordinately noticeable in the image. In performing monochrome printing, therefore, it is desirable that an amount of chromatic color inks to be used are minimized as much as possible.
An attempt has been also made to improve image quality by mounting on the apparatus a plurality of inks of achromatic colors with varying concentrations (gray ink or the like), instead of a plurality of inks of chromatic colors including cyan, magenta, yellow and the like, and rendering gradation of a monochrome image using the plurality of inks of achromatic colors with varying concentrations (see Japanese Patent Application Laid-open No. 2000-177150). In recent years, a number of apparatuses mounted with a plurality of inks with varying concentration of black have been also put on the market.
There may be cases, in which all gradations covering from a highlight portion to a maximum optical density portion (a solid area density portion) are printed using only ink (e.g., black ink in a monochrome photo tone image) that can create an output of the maximum optical density of a is basic tone color. In such cases, particularly in middle gradation, granular impression with the deviation in landing positions of dots is noticeable. For example, contrast in monochrome printing is higher than that in color printing, because in monochrome printing black ink is deposited on a white print medium. A portion of dots locally concentrated due to the deviation of the landing positions tends to become noticeable as rendered as black lines or the like.
The deviation in landing positions of dots may probably be generated by part-to-part variations in nozzle configurations occurring in manufacturing processes of ink jet print heads and noise components such as vibration of the apparatus during printing.
As described in the foregoing, the deviation in landing positions of dots tends to be more noticeable in middle gradations with a decreasing number of colors of ink used, as symbolized by the case using ink of a single color only. To state it another way, the more the number of colors of ink used, the more the total amount of ink applied to a predetermined area on the print medium. This results in ink coverage on the surface of the print medium becoming higher. On the other hand, if the number of colors of ink used becomes less, the total amount of ink applied to the predetermined area on the print medium becomes small, resulting in the link coverage becoming lower. The deviation in landing positions of dots in higher ink coverage does not substantially affect the image quality. If the deviation in landing positions of dots occurs in low ink coverage, however, the image quality is appreciably affected. This is because of the following reason. Specifically, in the condition having the low ink coverage, there is a greater likelihood that the color of the print medium itself will be visible as compared the condition having the high ink coverage. The deviation in dot landing positions then helps make the color of the printing medium itself look to cyclically vary. Moreover, in monochrome images, the deviation in dot landing positions becomes even more noticeable because of a higher contrast between the color of ink and the color of the print medium, in addition to the originally low ink coverage from printing using the black ink only.
Deviation in dot landing positions involved uniquely with a nozzle row is probably attributable to the deviation in dot landing positions actually occurring in a printing apparatus. This problem of the deviation in dot landing positions involved uniquely with the nozzle row is due to part-to-part variations in the manufacturing processes of the print head. The problem is due to ejection characteristics of each individual print head, such as the dot landing position, the amount of ink ejected, and the like. Other possible reasons for the deviation in dot landing positions include: effect from a satellite or ink droplets collaterally ejected in addition to the main ink droplets during ejection of ink droplets; and fluctuations in speed of the carriage during scanning.
The present invention thus identifies a problem of degraded image quality noticeable in a print mode using a relatively small number of colors of ink, such as the monochrome print mode, the problem being attributable to the deviation in dot landing positions occurring from characteristics unique to the nozzle row. In view of the foregoing problem based on a relation between the number of colors of ink used and degraded image quality, it is an object of the present invention to provide an ink jet printing apparatus and an ink jet printing method capable of producing an output of a print result of high image quality showing deviation in dot landing positions not noticeable even with a small number of colors of ink used for printing.